VOL-4224: Bump minor version post voltha-2.8 release tag

Change-Id: I12a0d8ad013a683929496fc428068c6c516870c4
diff --git a/VERSION b/VERSION
index ab0fa33..f8ee15f 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-5.0.5
+5.1.0-dev